1. Field of the Invention
This invention relates to a method for constructing a filter bank used for efficiently band-splitting digital signals, and a filter bank apparatus constructed by this method.
2. Description of the Related Art
Among conventional illustrative picture compression systems, there is the Joint Photographic Experts Group (JPEG) system standardized by the International Organization for Standardization (ISO). This JPEG system uses discrete cosine transform (DCT) as a transform system and is able to furnish good encoded or decoded images subject to allocation of a larger number of bits. However, if the number of bits is reduced to a more or less extent, block distortion proper to DCT appears significantly on encoded or decoded images, such that subjective deterioration in the image quality becomes outstanding.
In distinction from these systems, investigations are going on briskly in connection with systems configured for splitting image signals into plural bands by a filter device combined from high-pass filters and low-pass filters, known as a filter bank, and for carrying out encoding from one band to another. Among these systems, wavelet transform has no defect that block distortion becomes outstanding for high compression in contradistinction from DCT and, for this reason, wavelet transform is felt to prove a promising technique which possibly takes the place of DCT.
At present, in an electronic still camera or video movie, the International Standard system, exemplified by JPEG or the Moving Picture Experts Group (MPEG), is accepted as a picture compression system, and DCT is used as the transform system which operates as a key for compression.
Researches towards improving the efficiency of the encoding system, founded on the wavelet transform, and which possibly takes the place of DCT-based encoding system, are conducted energetically in many research institutes. As an international standard system for the next-generation still picture compression system, which may be said to follow the suit of JPEG, there is the JPEG-2000, for which investigations are proceeding by the ISO/IEC/JTC1/SC29/WG1, which is the same organization as JPEG. In this JPEG-2000, wavelet transform is scheduled to be accepted in place of the routinely used DCT in JPEG as the transform system which is to be the basis of image compression.
Meanwhile, the wavelet transform is the system in which image signals are split into plural bands by a filter device, termed a filter bank, combined from high-pass filters and low-pass filters. However, there has not been known at present means for efficiently constructing this filter bank. For implementing an encoding system which is founded on wavelet transform, the necessity for establishing means for constructing the filter bank enabling efficient band splitting of a variety of image signals is felt to be increasing in future, such that there is an increasing demand for the means for constructing the filter bank.
It is therefore an object of the present invention to provide a method for constructing the filter bank efficiently. It is another object of the present invention to provide a filter bank apparatus constructed by this filter bank constructing method.
In one aspect, the present invention provides a filter bank constructing method and apparatus for constructing a band-splitting filter bank for splitting the spectrum of an input signal into a high frequency range and a low frequency range, in which low-pass analysis filter means(H0(z)), adapted for passing frequency components of the input signal not higher than a pre-set frequency, is determined by high pass analysis filter means (H1(z)), adapted for passing frequency components of the input signal not lower than the pre-set frequency, and by phase-linear filter means (f(z)) having phase linear characteristics.
In the filter bank constructing method and apparatus, according to the present invention, the low-pass analysis filter means is determined by determining the high pass analysis filter means and by the phase-linear filter means.
In another aspect, the present invention provides a method and apparatus for constructing a splitting filter bank in which a first signal obtained on downsampling an input signal (even-numbered component signal) is inputted to prediction filter means (P(z)), a second signal obtained on downsampling the input signal (odd-numbered component signal) and a prediction signal generated by the prediction filter means are inputted to subtraction means to generate a difference signal which is outputted as a high range component output signal, an updated signal generated on inputting the difference signal to updating filter means (U(z)) is summed to the first signal by summing means to output the sum signal as a low range component output signal. The prediction filter means is determined by phase-linear low-pass filter means having an odd number of orders of the filter coefficients (H1xe2x80x2(z)) and an even-number symmetrical impulse response, and the updating filter means is determined by phase-linear filter means having linear phase characteristics (f(z)).
In the method and apparatus for constructing a splitting filter bank, according to the present invention, the prediction filter means and the updating filter means are determined by determining the phase-linear low-pass filter means and the phase-linear filter means, respectively.
In still another aspect, the present invention provides a filter bank constructing method and apparatus for constructing a synthesis filter bank which is fed with low-range signals and high-range signals split respectively by low-pass analysis filter means adapted for passing frequency components of original signals not higher than a pre-set frequency (H0(z)) and by high-pass analysis filter means adapted for passing frequency components of the original signals not lower than a pre-set frequency H1(z)), and which synthesizes the low-range signals and high-range signals. The low-pass synthesis filter means, fed with the low-range signals and adapted for passing frequency components not higher than a pre-set frequency (F0(z)), is determined by the high-pass analysis filter means, and the high-pass synthesis filter means, fed with the high-range signals and adapted for passing frequency components not lower than a pre-set frequency (F1(z)), is determined by the low-pass analysis filter means.
In the method and apparatus for constructing a splitting filter bank, according to the present invention, the low-pass synthesis filter means is determined based on the determined high-pass analysis filter means, whilst the high-pass synthesis filter means is determined based on the determined low-pass analysis filter means.
In yet another aspect, the present invention provides a filter bank constructing method and apparatus for constructing a synthesis filter bank in which a low-range component output signal, obtained on splitting an original signal, and a high-range component output signal, obtained on splitting the original signal, are inputted to updating filter means (U(z)), to yield an updated signal, which updated signal is inputted along with the low-range component output signal to subtraction means to yield a difference signal, which difference signal is inputted to prediction filter means P(z)) to yield a prediction signal, which prediction signal and the high-range component output signal are suited together by first summing means to yield a sum signal, which sum signal is suited to the difference signal by second summing means and outputted as an output signal. The prediction filter means is determined by phase-linear low-pass filter means having an odd number of orders of the filter coefficients and even-number symmetrical impulse response (H1xe2x80x2(z)), and the updating filter means is determined by phase-linear filter means having phase-linear characteristics ((f(z)).
In the method and apparatus for constructing a splitting filter bank, according to the present invention, the prediction filter means is determined by determining the phase-linear low-pass filter means, whilst the updating filter means is determined by determining the phase-linear filter means.
In the method and apparatus for constructing a splitting filter bank, according to the present invention, the low-pass analysis filter means(H0(z)), adapted for passing frequency components of the input signal not higher than a pre-set frequency, can be efficiently determined by the high pass analysis filter means (H1(z)), adapted for passing frequency components of the input signal not lower than the pre-set frequency, and by phase-linear filter means (f(z)) having phase linear characteristics, thus enabling the input signal to be wavelet-split efficiently.
Also, in the method and apparatus for constructing a splitting filter bank, according to the present invention, the prediction filter means (P(z)) for generating prediction signals in a lifting configuration can be efficiently determined by determining phase-linear low-pass filter means (H1xe2x80x2(z)) having an odd number of orders of the filter coefficients and an even-number symmetrical impulse response. Moreover, the updating filter means (U(z)) for generating updating signals in the lifting configuration can be determined efficiently by determining the phase-linear filter means (f(z)) having phase-linear characteristics, thus enabling the updating filter means (U(z)) generating updated signals in the lifting configuration to be determined efficiently.
Moreover, in the method and apparatus for constructing a splitting filter bank, according to the present invention, the low-pass synthesis filter means (F0(z)), adapted to pass frequency components of input low-range signals not larger than a pre-set frequency, can be automatically determined by determining high-pass analysis filter means (H1(z)), adapted for passing frequency components of the original signal not less than a pre-set frequency, whilst the high-pass synthesis filter means (F,(z)), adapted to pass frequency components of input high-range signals not larger than a pre-set frequency, can be automatically determined by determining low-pass analysis filter means (H0(z)), adapted for passing frequency components of the original signal not larger than the pre-set frequency, thus enabling wavelet synthesis.
In addition, in the method and apparatus for constructing a splitting filter bank, according to the present invention, the prediction filter means (P(z)), adapted for generating prediction signals in the lifting configuration, can be easily determined by determining the phase-linear low-pass filter means (H1xe2x80x2(z)) having an odd number of orders of filter coefficients and even-number symmetry of the impulse response, whilst the updating filter means (U(z)), adapted for generating updating signals in the lifting configuration, can be easily determined by determining the phase-linear filter means (f(z)) having phase-linear characteristics.